Three Jewish watchdog groups say they’ve long had their sights set on the elderly man accused of storming the U.S. Holocaust Museum and killing a guard Wednesday afternoon.

The suspect, whom authorities have identified as James W. von Brunn, operated an anti-Semitic Web site pushing a book purporting to be an “expose of the Jew conspiracy to destroy the white gene-pool.”

Rabbi Marvin Hier, founder and dean of the Simon Wiesenthal Center for Holocaust Studies in Los Angeles, said his group’s researchers have known of Mr. von Brunn’s site as one of “thousands” of anti-Semitic pages worldwide.

“It is a classic example of hatred,” the rabbi said. “It begins with speeches and writings and continues on from there. It’s a wake-up call to all of us, thinking this is America, and when we describe evil, we look for another country. But hatred exists in our own ranks and in our own country.”

The assailant “never hid his hatred,” the rabbi added. “He had an active Web site, where he said the greatest mistake Hitler made was not to gas all the Jews.”

Myrna Shinbaum, director of media relations for the Anti-Defamation League, said Mr. von Brunn “is not unknown in the world of people we look at.”

“We knew him,” she said. “He has a long history of these things, and he’s a hater of Jews. A lot of these people are on the fringe. They may not be known to you guys [in the media], which is why we need to know who they are.”

Ken Stern, a specialist on anti-Semites and extremism for the American Jewish Committee, said Mr. von Brunn’s name was known but he was “not a mover and a shaker” or one of the better-known names in the white-supremacist movement.

Still, he said, a lone wolf like Mr. von Brunn was apparently able to kill someone without being any kind of leader or having a supportive infrastructure.

“This is just another example of how people steeped in ideology of white supremacy and anti-Semitism can end up giving vent to their ideas in terroristic acts,” he said.

Officials at the Illinois Holocaust Museum and Education Center in Skokie beefed up their security Wednesday afternoon after the shooting in Washington.

The Wiesenthal Center already has extensive security. When white-supremacist Buford O. Furrow Jr. planned in summer 1999 to attack the museum, he was scared off by its security staff. He then went on a shooting rampage at a Los Angeles Jewish community center and killed a Filipino-American postal worker.

Mr. von Brunn’s site includes a quote from the Talmud, which is made up of second-century commentaries on Jewish scriptures, saying “Kill the Best Gentiles.”

Nathan Diament, policy director for the Union of Orthodox Jewish Congregations of America and a member of President Obama’s faith-based advisory council, said the passage was advice on how to defeat the Romans during a time when they heavily persecuted Jews.

It was advice linked to the parting of the Red Sea mentioned in Exodus 9:6, he said, and not applicable during later centuries.

Some Jewish leaders pointed out that President Obama’s recently visited a World War II German concentration camp.

“We are especially shocked that this happened less than one week after President Obama was in Buchenwald denouncing Holocaust deniers. This just reinforces the need for good citizens of all faiths to stand up to denounce hatred and bigotry in their community, and not just denounce it, but confront it, and not think it’s someone else’s problem,” said Ron Halber, executive director of the Jewish Community Relations Council of Greater Washington.

“Violent actors like this try to instill fear in Americans - in this case, American Jews - to change their daily lives and their patterns, but we are not going to give in to hate and fear,” he added.

Mr. Diament also brought up the president’s recent trip to Buchenwald as “a powerful response to the Holocaust.”

He added, “This [shooting] is both a tragic and offensive event; tragic, that innocent museum visitors were terrorized and that a brave security guard [was killed]; offensive, that someone would trespass upon a place where hate-inspired violence of the past is remembered for the sake of diminishing it in the present.”

Israeli Information and Diaspora Minister Yuli Edelstein told reporters in Jerusalem that the shooting was “further proof that anti-Semitism and Holocaust denial have not passed from the world.”

Also Wednesday, two Muslim groups issued statements denouncing the attack.

“We condemn this apparent bias-motivated attack and stand with the Jewish community and with Americans of all faiths in repudiating the kind of hatred and intolerance that can lead to such disturbing incidents,” said a release from the Council on American-Islamic Relations.

“The ADC is consistent in condemning hate-motivated crimes and any attacks aimed at civilians, no matter who the victims or the perpetrators may be,” he said.

Rabbi Eric Yoffie, president of the Union for Reform Judaism, said he was grateful that quick action by police and security guards prevented further violence.

“That today’s shooting at the United States Holocaust Museum should take place at a site expressly created to teach the world about the destruction and devastation brought about by human evil deepens the resonance of this terrible act,” he said.